72 THE -HABITAT- 
ings of the same leaf were .5°—1° higher when made upon the upper smooth 
surface. This immediately suggests that the lower surface may be modified 
to protect the leaf from the great heat of the gravel, which often reaches 
50° C. (122° F.). 
PRECIPITATION 
105. General relations. As the factor which exerts the most important 
control upon water-content and humidity, rainfall must be carefully con- 
sidered by the ecologist. It is ‘such an obvious factor, and is usually spoken 
of in such general terms that the need of following it accurately is not evi-- 
dent at once. When it is recognized that the fluctuations of water-content 
are directly traceable to it, it hecomes clear that its determination is as 
important as that of any indirect factor. This does not mean, however, 
that the amount of yearly rainfall is to be taken from the records of the 
nearest weather station, and the factor dismissed. Like other instruments, 
the rain gauge must be kept at the base station of the area under study, 
and when this is extensive or diverse, additional instruments should be 
put into commission. While the different parts of the same general cli- 
matic region may receive practically the same amount of precipitation dur- 
ing the year, it is not necessarily true that the rainfall of any particular 
storm is equally distributed, especially in the mountains. Nothing less 
than an. exact’ knowledge of the amount of rain that falls in the different 
areas will make it possible to tell how much of the water-content found at 
any particular time in these represents merely the chance differences of 
precipitation. : 
The forms of precipitation are rain, dew, hail, snow, and frost. Of 
these, hail is too infrequent to be taken into account, while frost .usually 
occurs only at the extremes of the growing season, and in its effect is 
rather to be reckoned with temperature. Snow rarely falls except during 
the period of rest, and, while it plays an important part as cover, it is 
merely one of several factors that determine the water-content of the soil at 
the beginning of spring. The influence of dew is not clearly understood. It 
is almost always too slight in amount and too fleeting to affect the water- 
content of the soil. It seems probable that it may serve by its own evapora- 
tion to decrease in some degree the. water loss from the soil, and from be- 
dewed plants. If, however, the dew is largely formed by the water of the 
soil and of the plant, as is thought by some, then it is negligible as a re- 
inforcement of water-content. From the above, it is evident that rainfall 
alone exerts a profound effect upon the habitat, and it is with its measure- 
ment that the ecologist is chiefly concerned. 
